Obesity is a life-threatening disorder in which there is an increased risk of morbidity and mortality arising from concomitant diseases such as, but not limited to, type II diabetes, hypertension, stroke, certain forms of cancers and gallbladder disease.
Obesity has become a major healthcare issue in the Western World and increasingly in some third world countries. The increase in the number of obese people is due largely to the increasing preference for high fat content foods but also, and this can be a more important factor, the decrease in activity in most people's lives. In the last 10 years there has been a 30% increase in the incidence of obesity in the USA and that about 30% of the population of the USA is now considered obese. In spite of the growing awareness of the health concerns linked to obesity the percentage of individuals that are overweight or obese continue to increase. In fact, the percentage of children and adolescents who are defined as overweight has more than doubled since the early 1970s and about 13 perceHnt of children and adolescents are now seriously overweight. The most significant concern, from a public health perspective, is that children who are overweight grow up to be overweight or obese adults, and accordingly are at greater risk for major health problems. Therefore, it appears that the number of individuals that are overweight or obese will continue to increase.
Whether someone is classified as overweight or obese is generally determined on the basis of his or her body mass index (BMI) which is calculated by dividing their body weight (kilograms—Kg) by their height squared (meters squared—m2). Thus, the units for BMI are Kg/m2. The BMI is more highly correlated with body fat than any other indicator of height and weight. A person is considered overweight when they have a BMI in the range of 25-30 kg/m2. Whereas a person with a BMI over 30 kg/m2 is classified as obese and obesity is further divided into three classes, Class I (BMI of about 30 to about 34.9 kg/m2), Class II (BMI of about 35 to 39.9 kg/m2) and Class III (about 40 kg/m2 or greater); see TABLE 1 below for complete classifications.
TABLE 1CLASSIFICATION OF WEIGHTBY BODY MASS INDEX (BMI)BMICLASSIFICATION<18.5Underweight18.5-24.9Normal25.0-29.9Overweight30.0-34.9Obesity (Class I)35.0-39.9Obesity (Class II)>40Extreme Obesity (Class III)As the BMI increases for an individual there is an increased risk of morbidity and mortality relative to an individual with normal BMI. Accordingly, overweight and obese individuals (BMI of about 25 kg/m2 and above) are at increased risk for physical ailments such as, but not limited to, high blood pressure, cardiovascular disease (particularly hypertension), high blood cholesterol, dyslipidemia, type II (non-insulin dependent) diabetes, insulin resistance, glucose intolerance, hyperinsulinemnia, coronary heart disease, angina pectoris, congestive heart failure, stroke, gallstones, cholescystitis and cholelithiasis, gout, osteoarthritis, obstructive sleep apnea and respiratory problems, some types of cancer (such as endometrial, breast, prostate, and colon), complications of pregnancy, poor female reproductive health (such as menstrual irregularities, infertility, irregular ovulation), diseases of reproduction (such as sexual dysfunction, both male and female, including male erectile dysfunction), bladder control problems (such as stress incontinence), uric acid nephrolithiasis, psychological disorders (such as depression, eating disorders, distorted body image, and low self esteem). Research has shown that even a modest reduction in body weight can correspond to a significant reduction in the risk of developing other ailments, such as, but not limited to, coronary heart disease.
As mentioned above, obesity increases the risk of developing cardiovascular diseases. Coronary insufficiency, atheromatous disease, and cardiac insufficiency are at the forefront of the cardiovascular complications induced by obesity. The incidence of coronary diseases is doubled in subjects less than 50 years of age who are 30% overweight. The diabetes patient faces a 30% reduced lifespan. After age 45, people with diabetes are about three times more likely than people without diabetes to have significant heart disease and up to five times more likely to have a stroke. These findings emphasize the inter-relations between risks factors for NIDDM and coronary heart disease and the potential value of an integrated approach to the prevention of these conditions based on the prevention of obesity [Perry, I. J., et al. BMJ 310, 560-564 (1995)]. It is estimated that if the entire population had an ideal weight, the risk of coronary insufficiency would decrease by 25% and the risk of cardiac insufficiency and of cerebral vascular accidents by 35%.
Diabetes has also been implicated in the development of kidney disease, eye diseases and nervous-system problems. Kidney disease, also called nephropathy, occurs when the kidney's “filter mechanism” is damaged and protein leaks into urine in excessive amounts and eventually the kidney fails. Diabetes is also a leading cause of damage to the retina and increases the risk of cataracts and glaucoma. Finally, diabetes is associated with nerve damage, especially in the legs and feet, which interferes with the ability to sense pain and contributes to serious infections. Taken together, diabetes complications are one of the nation's leading causes of death.
The first line of treatment for individuals that are overweight or obese is to offer diet and life style advice, such as, reducing the fat content of their diet and increasing their physical activity. However many patients find these difficult to maintain and need additional help from drug therapy to sustain results from these efforts.
Most currently marketed products have been unsuccessful as treatments for obesity owing to a lack of efficacy or unacceptable side-effect profiles. The most successful drug so far was the indirectly acting 5-hydroxytryptamine (5-HT) agonist d-fenfluramine (Redux™) but reports of cardiac valve defects in up to one third of the patient population led to its withdrawal by the FDA in 1998.
In addition, two drugs have recently been launched in the USA and Europe: Orlistat (Xenical™), a drug that prevents absorption of fat by the inhibition of pancreatic lipase, and Sibutramine (Reductil™), a 5-HT/noradrenaline re-uptake inhibitor. However, side effects associated with these products may limit their long-term utility. Treatment with Xenical™ is reported to induce gastrointestinal distress in some patients, while Sibutramine has been associated with raised blood pressure in some patients.
Serotonin (5-HT) neurotransmission plays an important role in numerous physiological processes both in health and in psychiatric disorders. 5-HT has been implicated in the regulation of feeding behavior for some time. 5-HT works by inducing a feeling of fullness or satiety so eating stops earlier and fewer calories are consumed. It has been shown that a stimulatory action of 5-HT on the 5HT2C receptor plays an important role in the control of eating and in the anti-obesity effect of d-fenfluramine. As the 5HT2C receptor is expressed in high density in the brain (notably in the limbic structures, extrapyramidal pathways, thalamus and hypothalamus i.e. PVN and DMH, and predominantly in the choroid plexus) and is expressed in low density or is absent in peripheral tissues, a selective 5HT2C receptor agonist can be an effective and safe anti-obesity agent. Also, 5HT2C knockout mice are overweight with cognitive impairment and susceptibility to seizure thus establishing the clear use for a 5HT2C receptor agonist in 5HT2C receptor associated diseases or disorders.
The 5HT2C receptor plays a role in obsessive compulsive disorder, some forms of depression, and epilepsy. Accordingly, 5HT2C receptor agonists can have anti-panic properties, and properties useful for the treatment of sexual dysfunction. In addition, 5HT2C receptor agonists are useful for the treatment of psychiatric symptoms and behaviors in individuals with eating disorders such as, but not limited to, anorexia nervosa and bulimia nervosa. Individuals with anorexia nervosa often demonstrate social isolation. Anorexic individuals often present symptoms of being depressed, anxious, obsession, perfectionistic traits, and rigid cognitive styles as well as sexual disinterest. Other eating disorders include, anorexia nervosa, bulimia nervosa, binge eating disorder (compulsive eating) and ED-NOS (i.e., eating disorders not otherwise specified—an official diagnosis). An individual diagnosed with ED-NOS possess atypical eating disorders including situations in which the individual meets all but a few of the criteria for a particular diagnosis. What the individual is doing with regard to food and weight is neither normal nor healthy.
In addition, the 5HT2C receptor is also involved in other diseases, conditions and disorders; such as Alzheimer Disease (AD). Therapeutic agents currently prescribed for Alzheimer's disease (AD) are cholinomimetic agents that act by inhibiting the enzyme acetylcholinesterase. The resulting effect is increased levels of acetylcholine, which modestly improves neuronal function and cognition in patients with AD. Although, dysfunction of cholinergic brain neurons is an early manifestation of AD, attempts to slow the progression of the disease with these agents have had only modest success, perhaps because the doses that can be administered are limited by peripheral cholinergic side effects, such as tremors, nausea, vomiting, and dry mouth. In addition, as AD progresses, these agents tend to lose their effectiveness due to continued cholinergic neuronal loss.
Therefore, there is a need for agents that have beneficial effects in AD, particularly in alleviating symptoms by improving cognition and slowing or inhibiting disease progression, without the side effects observed with current therapies. Therefore, serotonin 5HT2C receptors, which are exclusively expressed in brain, are attractive targets.
A major feature of AD is the formation of senile plaques made of amyloid deposits in a selected area of the brain. New therapies should focus on prevention of the production of these senile plaques. An amyloid deposit composed mainly of beta-amyloid peptide (Aβ) occupies the plaque center. Aβ is a peptide of 40 to 43 residues derived from a larger amyloid precursor protein, APP [Selkoe D J, et al. Ann Rev Neurosci, 1994, 17:489-517]. APP is a ubiquitous transmembrane glycoprotein that is present at high levels in brain cells. APP also exists as secreted forms. By cleavage in the Aβ region of APP, the long N-terminal fragment (secreted APP, APPs) is secreted into the extracellular space. The rate of Aβ production appears to be inversely coupled to rate APPs secretion. In several cell cultures, APPs secretion was accompanied by reductions in secreted Aβ [Buxbaum J D, et al. Proc Nat Acad Sci, 1993, 90:9195-9198; Gabuzda D, et al. J Neurochem, 1993, 61:2326-2329; Hung A Y, et al. J Biol Chem, 1993, 268:22959-22962; and Wolf B A, et al. J Biol Chem, 1995, 270:4916-4922], suggesting that stimulated secretory processing of APP into secreted APPs is associated with reduced formation of potentially amyloidogenic derivatives, or plaques.
APPs is found in plasma and cerebrospinal fluid [Ghiso J, et al. Biochem Biophys Res Comm, 1989, 163:430-437; and Podlisny M B, et al. Biochem Biophys Res Commun, 1990, 167:1094-1101]. Considering the abundance of both membrane-bound APP and APPs, they are likely to have significant biological functions. Current knowledge about APP functions indicates APP is critically required for the maintenance of neuronal and synaptic structure and function. Membrane-bound APP has been suggested to have a receptor-like structure [Kang J, et al. Nature, 1987, 325:733-736], with the cytoplasmic domain capable of complexing with a GTP-binding protein [Nishimoto I., et al. Nature, 1993, 362:75-79]. Membrane-embedded full-length APP might also have a cell adhesion function [Qiu W., et al. J Neurosci, 1995, 15:2157-2167].
APPs has been shown to be neurotrophic and neuroprotective in vitro [Mattson M P, et al. Neuron, 1993, 10:243-254; and Qiu W., et al. J Neurosci, 1995, 15:2157-2167]. Other proposed functions for APPs include the regulation of blood coagulation [Cole G M, et al. Biochem Biophys Res Commun, 1990, 170:288-295; Smith R P, et al. Science, 1990, 248:1126-1128; and Van Nostrand et al. Science, 1990, 248:745-748], wound-healing [Cunningham J M, et al. Histochemistry, 1991, 95:513-517], extracellular protease activity [Oltersdorf T, et al. Nature (London), 1989, 341:144-147; and Van Nostrand W E, et al. Nature, 1989, 341:546-548], neurite extension [Jin L., et al. J Neurosci, 1994, 14:5461-5470; and Robakis N K, et al. in Molecular Biology of Alzheimer's Disease. (T. Miyatake, D. J. Selkoe and Y. Ihara, ed.), 1990, pp. 179-188,  Elsevier Science Publishers B.V., Amsterdam], cell adhesiveness [Schubert D, et al. Neuron, 1989, 3:689-694], cell growth, [Bhasin R., et al. Proc Natl Acad Sci USA, 1991, 88:10307-10311; and Saitoh T., Cell, 1989, 58:615-622], and differentiation [Araki W., et al. Biochem Biophys Res Commun, 1991, 181:265-271; Milward E A, et al. Neuron, 1991, 9:129-137; and Yamamoto K, et al. J Neurobiol, 1994, 25:585-594].
The non-selective serotonin 5HT2C agonist dexnorfenfluramine (DEXNOR) stimulated amyloid precursor protein (APPs) secretion in guinea pigs while reducing levels of Aβ production in vivo following repeat administration [Arjona A, et al. “Effect of a 5HT2C serotonin agonist, dexnorfenfluramine, on amyloid precursor protein metabolism in guinea pigs,” Brain Res, 2002, 951:135-140]. Guinea pigs were chosen because guinea pig and human APP exhibit 98% sequence homology [Beck M, et al. Biochem Biophys Acta, 1997, 1351:17-21], the proteins are processed similarly [Beck M., et al. Neuroscience, 1999, 95:243-254], and the Aβ peptide sequences are identical [Johnstone E M, et al. Brain Res Mol Brain Res, 1991, 10:299-305]. Although DEXNOR is non-selective, the observed effects were attenuated by a selective serotonin 5HT2C antagonist, while a selective serotonin HT2A antagonist did not reverse the DEXNOR effects, indicating the serotonin 5HT2C receptors are the most relevant target for this effect.
In addition, 5-HT stimulates APPs ectodomain secretion via the serotonin 5HT2A and 5HT2C receptors [Nitsch R M, et al. J Biol Chem, 1996, 271(8):4188-4194]. In this study, researchers stimulated 3T3 fibroblasts with serotonin (5-HT), which were stably expressing serotonin 5HT2A or 5HT2C receptors. 5-HT increased APPs secretion in a dose-dependent manner in both cell lines. Maximal stimulation of APPs secretion peaked at about 4-fold. Selective serotonin 5HT2A and 5HT2C antagonists blocked the effects in each cell line.
A serotonin 5HT2C receptor agonist can be effective for treating AD and preventing senile plaques. Support for this claim comes from the fact that Aβ is known to be neurotoxic and a key component in senile plaques involved in AD, APPs secretion and Aβ levels seem to be inversely related, and serotonin 5HT2C agonists increase levels of APPs in vitro in cell lines stably expressing serotonin 5HT2C receptors while in vivo serotonin 5HT2C agonists increase levels of APPs and decrease levels of Aβ as measured in cerebral spinal fluid of guinea pigs.
Evidence exists supporting the use of a compound of the present invention with agonist activity at the serotonin 5HT2C receptor for the treatment of AD. The compound of the invention can be used alone or in combination with another agent or agents (such as but not limited to AChE inhibitors) that are typically prescribed for AD.
Another disease, disorder or condition that can is associated with the function of the 5HT2C receptor is erectile dysfunction (ED). Erectile dysfunction is the inability to achieve or maintain an erection sufficiently rigid for intercourse, ejaculation, or both. An estimated 20-30 million men in the United States have this condition at some time in their lives. The prevalence of the condition increases with age. Five percent of men 40 years of age report ED. This rate increases to between 15% and 25% by the age of 65, and to 55% in men over the age of 75 years.
Erectile dysfunction can result from a number of distinct problems. These include loss of desire or libido, the inability to maintain an erection, premature ejaculation, lack of emission, and inability to achieve an orgasm. Frequently, more than one of these problems presents themselves simultaneously. The conditions may be secondary to other disease states (typically chronic conditions), the result of specific disorders of the urogenital system or endocrine system, secondary to treatment with pharmacological agents (e.g. antihypertensive drugs, antidepressant drugs, antipsychotic drugs, etc.) or the result of psychiatric problems. Erectile dysfunction, when organic, is primarily due to vascular irregularities associated with atherosclerosis, diabetes, and hypertension.
There is evidence for use of a serotonin 5HT2C agonist for the treatment of sexual dysfunction in males and females. The serotonin 5HT2C receptor is involved with the processing and integration of sensory information, regulation of central monoaminergic systems, and modulation of neuroendocrine responses, anxiety, feeding behavior, and cerebrospinal fluid production [Tecott, L. H., et al. Nature 374: 542-546 (1995)]. In addition, the serotonin 5HT2C receptor has been implicated in the mediation of penile erections in rats, monkeys, and humans.
The exact mechanism by which 5HT2C receptors mediate penile erections remains unknown. However, there is good evidence, indirect and direct, supporting the role of serotonin 5HT2C receptors in the mediation of penile erections. Anatomical studies have shown that the penis receives autonomic innervation from sympathetic and parasympathetic nuclei located in the spinal cord [Pescatori E S, et al. J Urol 1993; 149: 627-32]. In agreement, experimental and clinical data support that penile erections are controlled by a spinal reflex. A closer analysis showed that activation of 5HT2 spinal receptors facilitated pudendal reflex in anesthetized cats [Danuser H and Thor K B, Br J Pharmacol 1996; 118: 150-4]. Accordingly, stimulation of 5HT2C receptors has been shown to be proerectile [Millan M J, et al. European Journal of Pharmacology 1997; 325], and 5HT2C receptors have been described on proerectile spinal parasympathetic neurons [Bancila M et al. Neuroscience 1999; 92: 1523-37].
Indirect evidence comes from the research and reports of the side effects induced by the use of selective serotonin reuptake inhibitors (SSRIs). SSRIs have demonstrated antagonist action at the serotonin 5HT2C receptors [Jenck et al. European Journal of Pharmacology 231: 223-229 (1993); Lightlowler et al. European Journal of Pharmacology 296: 137-43 (1996); and Palvimaki, E., et al. Psychopharmacology 126: 234-240 (1996)]. Among the most derogatory side effects of SSRIs noted in humans is increased difficulty in attaining penile erection. Although SSRIs have a rich pharmacological profile, it is believed that the antagonist effects of SSRIs at the 5HT2C receptors could be implicated in the inhibition of penile erections [Palvimaki, E., et al. Psychopharmacology 126: 234-240 (1996)].
Further evidence comes from studies with a variety compounds with known agonist activity for the serotonin 5HT2C receptor. Pharmacologic studies with rats and rhesus monkeys provide direct evidence of the proerectile properties of agonist of the serotonin 5-HT2C receptor [Millan M J, et al. European Journal of Pharmacology 1997; 325; and Pomerantz, et al. European Journal of Pharmacology 243:227-34 (1993)]. These pro-erectile effects were unaffected by antagonists for the serotonin 5HT2A and 5HT2B receptors, respectively. Antagonists of the serotonin 5HT2C receptors attenuated the proerectile effects of the 5-HT2C agonists. The inhibition action corresponded to each antagonist's affinity for the 5-HT2C receptors. In addition, agonists of the serotonin 5HT2A and 5HT2B receptors did not elicit penile erections.
In summary, the 5HT2C receptor is a validated and well-accepted receptor target for the prophylaxis and/or treatment of 5HT2C mediated receptor diseases and disorders, such as, obesity, eating disorders, psychiatric disorders, Alzheimer Disease, sexual dysfunction and disorders related thereto. It can be seen that there exists a need for selective 5HT2C receptor agonists that can safely address these needs. The present invention is directed to these, as well as other, important ends.